In a multiple-input, multiple-output (MIMO) communication system using wireless orthogonal frequency division multiplexing (OFDM), the use of multiple antennas is much preferred in order to increase the performance of the system. When multiple antenna transmission at the transmitter side, either the base station or the mobile station, is used, then the space dimension can be exploited for scheduling of the transmission to the different users in the system.
To increase the throughput of the cell, an OFDM time-frequency resource can simultaneously be shared by multiple users. This is described in publications “Philips, 3GPP Technical document R1-071403”, “Samsung, 3GPP Technical document R1-060335”, “On the optimality of multiantenna broadcast scheduling using zero forcing beamforming” by T. Yoo and A. Goldsmith, IEEE journal on selected areas in communications, Vol. 24, No. 3, March 2006, pp. 528-541.
When two or more users share the same time-frequency resource, the transmissions to the co-scheduled users will mutually interfere with each other at the receiver side.
To minimize this interference between the co-scheduled users, precoding at the base station is performed in form of a linear mapping of each users signal to the transmitter antennas. This linear mapping is easily described by a matrix multiplication, where a precoding matrix, is jointly selected by the scheduled mobile users to minimize the mutual interference. Hence, precoded MU-MIMO is a closed loop MIMO scheme where the transmission is adjusted based on the channel to the co-scheduled mobile users.
In more detail, each mobile user selects a channel quantization vector (CQV) from a finite mobile user codebook of precoding vectors. The CQV is a quantization of the estimated channel vector from the NT base station transmitter antennas to the receiver antenna at the mobile user. If the mobile user has multiple receive antennas, the mobile user can find a receiver filter to combine the multiple receive antennas to obtain a virtual single antenna. The CQV is then estimated as the channel from the NT base station transmitter antennas to the single virtual receive antenna, i.e. after receiver filtering.
The selected CQV from the mobile user codebook is signalled to the base station. The scheduler in the base station then determines which mobile users are to be co-scheduled in an OFDM time frequency resource and selects the precoding matrix based on the reported CQV from these users.
For correct reception and demodulation of the transmitted signals, the mobile users need to have knowledge of the used transmitter precoding matrix and this information needs to be signalled to the mobile users.
Furthermore, knowledge of the full precoding matrix at the mobile user enables the use of interference suppression algorithms to minimize the residual interference from co-scheduled mobile users.
In “Philips, 3GPP Technical document R1-071403”, a method to solve the large signalling overhead problem is discussed where it is suggested to use dedicated precoded reference signals, one set of reference signals per scheduled mobile user. The overhead of this method is compared with the overhead of signalling the used base station precoding matrix index. However, no attempt was made in the comparison to reduce the signalling overhead in the downlink from the upper limit value of log2(KL) bits where K is the user codebook size and L is the number of co-scheduled users.
One problem with MU-MIMO signalling lies in the overhead of signalling the used precoding matrix at the base station transmitter to all the scheduled mobile users. If L mobile users are scheduled together in a mutual OFDM time-frequency resource, and there is a one-to-one correspondence between the selected vectors in the mobile user codebook and the selected base station precoding matrix, then there are KL possible base station transmitter precoding matrices where K is the number of vectors in the mobile user codebook. To signal the used transmitter precoding matrix in the downlink, L·log2(K) bits are needed, which generally is too large to be practical.